LIGO Scientific Collaboration, Virgo Collaboration

We present the results from three gravitational-wave searches for coalescing compact binaries with component masses above 1$\mathrm{M}_\odot$ during the first and second observing runs of the Advanced gravitational-wave detector network. During the first observing run (O1), from September $12^\mathrm{th}$, 2015 to January $19^\mathrm{th}$, 2016, gravitational waves from three binary black hole mergers were detected. The second observing run (O2), which ran from November $30^\mathrm{th}$, 2016 to August $25^\mathrm{th}$, 2017, saw the first detection of gravitational waves from a binary neutron star inspiral, in addition to the observation of gravitational waves from a total of seven binary black hole mergers, four of which we report here for the first time: GW170729, GW170809, GW170818 and GW170823. For all significant gravitational-wave events, we provide estimates of the source properties. The detected binary black holes have total masses between $18.6_{-0.7}^{+3.1}\mathrm{M}_\odot$, and $85.1_{-10.9}^{+15.6} \mathrm{M}_\odot$, and range in distance between $320_{-110}^{+120}$ Mpc and $2750_{-1320}^{+1350}$ Mpc. No neutron star - black hole mergers were detected. In addition to highly significant gravitational-wave events, we also provide a list of marginal event candidates with an estimated false alarm rate less than 1 per 30 days. From these results over the first two observing runs, which include approximately one gravitational-wave detection per 15 days of data searched, we infer merger rates at the 90% confidence intervals of 110 - 3840 $\mathrm{Gpc}^{-3}\,\mathrm{y}^{-1}$ for binary neutron stars and 9.7 - 101 $\mathrm{Gpc}^{-3}\,\mathrm{y}^{-1}$ for binary black holes, and determine a neutron star - black hole merger rate 90% upper limit of 610 $\mathrm{Gpc}^{-3}\,\mathrm{y}^{-1}$.

LIGO Scientific Collaboration, Virgo Collaboration

We present results on the mass, spin, and redshift distributions of the ten binary black hole mergers detected in Advanced LIGO's and Advanced Virgo's first and second observing runs. We constrain properties of the binary black hole (BBH) mass spectrum using models with a range of parameterizations of the BBH mass and spin distributions. We find that the mass distribution of the more massive black hole in each binary is well approximated by models with almost no black holes larger than 45 \Msol, and a power law index of $\alpha = \ALPHAMimrpNoSpinVTQuadBpm$ (90\% credibility). We also show that BBHs are unlikely to be composed of black holes with large spins aligned to the orbital angular momentum. Modelling the evolution of the BBH merger rate with redshift, we show that it is increasing with redshift with credibility $\FRACLambdaGTZero{}$. Marginalizing over uncertainties in the BBH population, we find robust estimates of the BBH merger rate density of $R = \RATEimrpNoSpinVTQuadCpm$ \invstvol (90\% credibility). As the BBH catalog grows in future observing runs, we expect that uncertainties in the population model parameters will shrink, potentially providing insights into the formation of black holes via supernovae, binary interactions of massive stars, stellar cluster dynamics, and the formation history of black holes across cosmic time.

Chrysoula Markou, Felix J. Rudolph, Angnis Schmidt-May

This work proposes a new gravitational theory formulated in terms of the vierbein field. The vierbein contains components which can be shifted by local Lorentz transformations and therefore do not show up in the spacetime metric. These components are given dynamics and become physical in our setup. They enter the massless theory in the form of an antisymmetric tensor field which makes the action reminiscent of the bosonic sector of supergravity. We then demonstrate that both the metric and the antisymmetric tensor can be made massive by adding a potential term for the vierbein. The form of this mass potential is inspired by ghost-free massive gravity. We confirm the absence of additional and potentially pathological degrees of freedom in an ADM analysis.

Stefano Anselmi, Pier-Stefano Corasaniti, Ariel G. Sanchez, Glenn D. Starkman, Ravi K. Sheth, Idit Zehavi

Leveraging the Baryon Acoustic Oscillations (BAO) feature present in clustering 2-point statistics, we aim to measure cosmological distances independently of the underlying background cosmological model. However this inference is complicated by late-time non-linearities that introduce model and tracer dependencies in the clustering correlation function and power spectrum, which must be properly accounted for. With this in mind, we introduce the "Purely-Geometric-BAO," which provides a rigorous tool to measure cosmological distances without assuming a specific background cosmology. We focus on the 2-point clustering correlation function monopole, and show how to implement such an inference scheme employing two different methodologies: the Linear Point standard ruler (LP) and correlation-function model-fitting (CF-MF). For the first time we demonstrate how, by means of the CF-MF, we can measure very precisely the sound-horizon/isotropic-volume-distance ratio, $r_{d}/D_{V}(\bar{z})$, while correctly propagating all the uncertainties. Using synthetic data, we compare the outcomes of the two methodologies, and find that the LP provides up to $50\%$ more precise measurements than the CF-MF. Finally, we test a procedure widely employed in BAO analyses: fitting the 2-point function while fixing the cosmological and the non-linear-damping parameters at fiducial values. We find that this underestimates the distance errors by nearly a factor of $2$. We thus recommend that this practice be reconsidered, whether for parameter determination or model selection.